Overfull \hbox in Longtable

Question

I'm trying to get rid of the error in my Longtable

Overfull \hbox (1.19998pt too wide) in alignment at lines 6--10

This is my MWE

\documentclass[12pt,a4paper,twoside]{report}
\usepackage{longtable}
\usepackage{array}
\begin{document}

\begin{longtable}{|>{\raggedright\arraybackslash}p{\dimexpr 0.5\textwidth -2\tabcolsep\relax}|>{\raggedright\arraybackslash}p{\dimexpr 0.5\textwidth -2\tabcolsep\relax}|}
\hline
Test & Test\\
\hline
\end{longtable}

\end{document}

Why do I get this error? In my understanding both columns of the longtable should take exactly 50% of the textwidth minus the left and right tabcolsep.

Answer 1

In the calculation of the usable columnwidth, you are correctly substracting the whitespace padding. However, you have failed to take into account the widths of the three vertical rules that are present in the table as well. The width of these vertical rules is governed by the parameter \arrayrulewidth; its default value is 0.4pt in most document classes. Observe that 3*0.4pt=1.2pt, which is extremely close to the reported overfullness of 1.19998pt. (In fact, 1.19998 and 2 are the same up to a rounding error.)

What to do? Piece of cake! Just change both instances of

p{\dimexpr 0.5\textwidth -2\tabcolsep\relax

to

p{\dimexpr 0.5\textwidth -2\tabcolsep -1.5\arrayrulewidth\relax

Why 1.5\arrayrulewidth? Because 2*1.5 = 3 = # of vertical rules in the longtable.